This invention relates generally to automatic test equipment (ATE) for electronics, and, more particularly, to electronic sources that automatically switch between voltage control and current control depending upon programming and load conditions.
ATE systems commonly include a variety of electronic sources for setting bias conditions and testing DC characteristics of devices. A type of electronic source known as a xe2x80x9cV/Ixe2x80x9d source combines both voltage-forcing and current-forcing modes in a single instrument. The two modes generally share a common control circuit and output stage, but employ different feedback paths. The different feedback paths can be engaged programmatically, for example by explicitly setting a force-voltage or force-current mode, or can be engaged automatically, as described below.
Automatically controlled V/I sources accept programmed values for voltage and current, and switch between voltage-controlled and current-controlled modes automatically, as required, to ensure that neither the programmed voltage nor the programmed current is exceeded. For example, an automatically controlled V/I source programmed for 5 Volts and 5 mA would operate in voltage-controlled mode (at 5 Volts) when connected to grounded loads greater than 1 Kxcexa9, but would automatically switch to current-controlled mode (at 5 mA) when driving grounded loads less than 1 Kxcexa9.
V/I sources often include greater than two control modes. One type of V/I source provides one current-controlled mode and two voltage-controlled modes. This type of source functions as a current source with positive and negative voltage clamps. Another type of V/I source provides one voltage-controlled mode and two current-controlled modes. This type of source functions as a voltage source with positive and negative current limits. Some V/I sources include four feedback modesxe2x80x94two current-controlled modes and two voltage-controlled modes. Only three modes are allowed to be active at a time. The source can be used as either a current source with two clamps or a voltage source with two current limits, depending upon how the source is programmed.
FIG. 1 is a simplified illustration of a conventional V/I source 100. The source 100 is configured as a voltage source with two current limits. A digital-to-analog converter (DAC) 110 establishes a desired output voltage for the V/I source 100. A summing circuit 116 subtracts a voltage feedback signal from an output voltage of the DAC 110, to produce an error voltage, VERROR. A crossover circuit 122 selects one feedback path for passage to a control circuit 124. When the V/I source 100 is operating in voltage-controlled mode, the crossover circuit 122 passes the VERROR to the control circuit 124. The output of the control circuit 124 is fed to a gain stage 126 and to a shunt 128 before arriving at a device under test (DUT) 132. When operating in voltage-controlled mode, the V/I source 100 forms a closed loop feedback system among the elements described above, and maintains an output voltage at the DUT 132 at the value prescribed by the DAC 110.
The V/I source 100 also includes DACs 112 and 114 for establishing positive and negative current limits, respectively. A differential circuit 130 coupled to the shunt 128 produces a current feedback signal proportional to the voltage across the shunt. Summers 118 and 120 subtract the current feedback signal from the outputs of DACs 112 and 114, to develop error signals IPosError and INegError, respectively. When the V/I source 100 operates in positive current-controlled mode, the crossover circuit 122 passes IPosError to the control circuit 124; when it operates in negative current-controlled mode, the crossover circuit 122 passes INegError. The circuit elements combine to form feedback systems that maintain the output current of the V/I source 100 at the value prescribed by DAC 112 or DAC 114, depending upon which of the two current modes is operative.
The control circuit 124 typically includes an integrating circuit for establishing dominant frequency characteristics of the V/I source. The gain stage 126 may provide voltage gain, current gain, or both. The shunt 128 generally includes an array of different resistors that can be individually selected to accommodate different current ranges.
FIG. 2 illustrates a conventional crossover circuit 122 commonly used with the V/I source 100 of FIG. 1. The crossover circuit 122 includes operational amplifiers (op amps) 214 and 224, buffers 212 and 222, diodes 216, 218, 226, and 228, and resistors 210, 220, and 230. The op amps 214 and 224 each have two distinct states of operationxe2x80x94an active state and an inactive state.
Taking the op amp 214 as an example, the op amp 214 assumes the active state whenever IPosError is less than VERROR. Under these conditions, diode 216 becomes reverse-biased and diode 218 conducts in the forward direction. A feedback loop is formed consisting of op amp 214, diode 218, buffer 222, and resistor 220. The feedback loop tends to drive the input of the buffer 212 to a level equal to IPosError. The buffer 212 then provides IPosError to the input of the control circuit 124.
Op amp 214 assumes the inactive state whenever IPosError is greater than VERROR. Diode 218 becomes reverse-biased and diode 216 conducts. IPosError is thus cut off from the control circuit 124, and feedback is closed locally around op amp 214 via diode 216.
The negative polarity operates in an analogous manner. Op amp 224 assumes the active state whenever INegError is greater than VERROR. A feedback loop is formed consisting of op amp 224, diode 228, buffer 222, and resistor 230. The loop tends to drive the input of the buffer 212 to INegError, and establishes the negative current-controlled mode. When INegError is less than VERROR, diode 228 becomes reverse-biased and diode 226 conducts, thus cutting off INegError from the control circuit 124 and locally closing feedback around the op amp 224.
The crossover circuit 122 thus engages a current-controlled mode when either of the op amps 214 and 224 operates in its active state. When both op amps operate in their inactive states, the crossover circuit 122 engages voltage-controlled mode. Voltage-controlled mode is thus engaged whenever VERROR is greater than INegError and less than IPosError. In voltage-controlled mode, the crossover circuit 122 passes VERROR to the control circuit 124 via resistor 210.
The crossover circuit 122 generally operates smoothly and accurately, making virtually seamless transitions between feedback modes. We have recognized, however, that the crossover circuit 122 can behave improperly during programming and output transients. For example, when programming a fast voltage step (via the DAC 110), the signal VERROR undergoes a voltage step from its steady-state value. The resulting step can momentarily cause VERROR to cross IPosError or INegError. These conditions cause the V/I source to inappropriately switch from voltage-controlled mode to one of its current-controlled modes. The mode change is xe2x80x9cinappropriatexe2x80x9d because it is not caused by excessive current flow; in fact, the output current may be zero. Rather, it is the natural consequence of applying a fast programming step.
FIG. 3 illustrates this condition. FIG. 3 is a V/I plot of the V/I source 100 operating with the crossover circuit of FIG. 2. The curve 300 represents the output of the V/I source during its three distinct feedback modes:
1. The upper, horizontal portion of the curve 300 represents the output of the V/I source when the positive current loop is engaged and programmed to a value IProgPos;
2. The vertical portion represents the output when the voltage loop is engaged and programmed to a value VProg; and
3. The lower, horizontal portion represents the output when the negative current loop is engaged and programmed to a value IProgNeg.
As long as the V/I source 100 operates in a settled state, the output of the V/I source will fall somewhere along the V/I curve 300. This is not the case, however, during programming and output transients. As indicated above, the V/I source 100 assumes positive current-controlled mode whenever IProgPos greater than VERROR and negative current-controlled mode whenever IProgNeg less than VERROR. The shaded regions depicted in the left and right portions of FIG. 3 respectively illustrate these areas. During transients, the V/I source can momentarily operate in the shaded regions, inappropriately engaging the current limits even though the output current is within limits. The V/I source can also fail to engage its current limits, even though the programmed currents are exceeded.
Inappropriately switching to current-controlled mode dramatically slows the settling time of the V/I source. Once the V/I source changes to a current-controlled mode, it remains in that mode until the error signal of the active current-control loop and VERROR again cross. Depending upon the values of IProgPos and IProgNeg, hundreds of microseconds may pass before the V/I source restores itself to voltage-controlled mode. This interval is exceedingly long compared with the normal settling time of the V/I source, i.e., when the current loops do not engage. In an automatic testing environment, delays in programming a V/I source translate directly to reduced testing throughput. A reduction in throughput detrimentally impacts testing efficiency.
What is needed is a crossover circuit for a V/I source that does not inappropriately switch feedback modes and consequently cause programming delays.
With the foregoing background in mind, it is an object of the invention to prevent an electronic source from assuming improper feedback modes during programming and output transitions.
To achieve the foregoing object, as well as other objectives and advantages, an improved crossover circuit is provided for use with an electronic source having a control circuit and providing at least one voltage-controlled mode and at least one current-controlled mode. The crossover circuit includes a selector and a measurement circuit. The selector and measurement circuits both receive a plurality of signals indicative of feedback voltage and feedback current of the electronic source. The measurement circuit monitors the plurality of signals and generates, in response to occurrences of predetermined events on the plurality of signals, at least one control signal. The selector operates in response to the at least one control signal, to select one of the plurality of signals for passage to the control circuit. A feedback loop is then formed for the electronic source employing the selected signal.